1. Field of the Invention
The present invention relates to an image processing apparatus and an image forming apparatus having an automatic color selection (ACS) function.
2. Description of the Related Art
Some image forming apparatuses have an ACS function. For example, a color image forming apparatus disclosed in Japanese Patent Application Laid Open No. 2004-104718 includes a unit that stores position information of an original conveying member within an original conveying device relative to an image input unit, to prevent an erroneous ACS determination when an image is input from a see-through type original feeder, and to determine a color of the original image suited to a recorded image to be output. A sensor, which is provided in front of the image input unit within the original conveying device, detects a position of an original relative to the image input unit. A color-area determining unit determines whether an area in an input image is chromatic or achromatic, based on input image signals. A determination inhibiting signal, which inhibits the color-area determining unit from making an erroneous determination, calculates a position of the original conveying member within the original conveying device relative to a conveyed original from the position information of the original conveying member within the original conveying device relative to the image input unit, sensor information detecting the position of the original, and a linear velocity in the sub scan direction. The determination inhibiting signal is generated when the original enters and leaves the original conveying member within the original conveying device.
Other related techniques are disclosed in Japanese Patent Application Laid Open Nos. H6-014205 and 2001-036752.
When making a determination in the ACS for a regular-sized original, a valid image size in the sub scan direction is defined, so that a chromatic detection area of the original is counted and a chromatic detection signal is masked. However, when the original is of an irregular size, or when originals of different sizes are mixed, an end of the original in the sub scan direction is not defined. Thus, a determination area cannot be set in advance. Furthermore, when the end edge of an original is lifted when passing through an automatic document feeder (ADF), input image data might change to chromatic color data even when the original only includes achromatic color data, which results in an erroneous detection. The image forming apparatus disclosed in Japanese Patent Application Laid Open No. 2004-104718 prevents an erroneous ACS determination when an image is input from the original feeder and determines a color of the original image suited to a recorded image to be output.
This conventional image forming apparatus counts the time from a conveying roller to a reading position, and generates a determination inhibiting signal for the corresponding area. However, variations occur when machine parts are assembled. Further, application specific integrated circuits (ASIC) require more input pins, which increases the cost.